http://www.waterlevels.gc.ca/C&A/netgraphs_e.html

Another possibility are the wind directions changing the water levels between the two times you observed the rock. If you checked the first time with predominantly west winds and the second time during winds out of the east, it might explain the scenario as the winds push the water from side to side over a large area. I notice this up in purgatory cove where our cottage is. Not sure if this is the case, just a possibility.

That being said; there's no doubt the water levels are falling, but I doubt it's 18 inches in six weeks.

Elias.